International Tables for Crystallography, Volume A: Space-group symmetry. M. I. Aroyo, ed. (2016). 6th ed. Chichester: Wiley, for the International Union of Crystallography. DOI: 10.1107/97809553602060000114.
This is the canonical reference for space groups. The pre-1980s Volume 1 is actually closer to what GSAS-II uses. If your library does not have a physical copy of Volume A, they may offer on-line access.
Symmetry in Crystallography, Understanding the International Tables, (2012) P. G. Radaelli, Oxford Publ. for the International Union of Crystallography. DOI: 10.1002/crat.201200325.
The above two books explain how to read the International Tables, Volume A.
Fundamentals of Crystallography. Third Edition (2011). C. Giacovazzo, H. L. Monaco, G. Artioli, D. Viterbo, M. Milaneso, G. Ferraris, G. Gilli, P. Gilli, G. Zanotti and M. Catti. Edited by C. Giacovazzo. IUCr Texts on Crystallography No. 15, IUCr/Oxford University Press.
This is a weighty, comprehensive, and modern book on crystallography. If you are going to buy anything, this is what I would recommend.
X-Ray Structure Determination: A Practical Guide. 2nd ed., (1989). Stout, George H., and Lyle H. Jensen. John Wiley & Sons.
I find this book easy to read, perhaps because I bought it for a course as an undergrad and have gone through it so many times. Anything in this book on experimental practice is decades out of date.
There is quite a wealth of symmetry information on various web sites.